Most minor and potentially major oil spills and blowouts are routinely avoided by paying close attention to the fluid pressures and flow conditions inside the well during routine drilling and workover procedures. The same basic equipment and materials that drilling crews rely on to control the well are the exact same equipment and materials that prevent oil spills and blowouts. Simply put, the loss of well control can lead to an oil spill or a blowout. Drillers, assistant drillers, drilling fluid (mud) engineers, mudloggers, and the derrickman or the rig hands called out to assist the mud engineer all have a hand in helping to maintain stable well pressures and optimal drilling mud and cuttings flow inside the well.

The drilling mud, the casing and casing cement, and the blowout preventers are the primary materials / equipment that prevent oil spills and blowouts. Automated alarms, pressure-relief valves and vents, and well bore shut-in systems that activate according to preset  thresholds are available, but these safety devices are sometimes bypassed or over-ridden by human operators. In such situations, the attention to the minute details of the well conditions on the part of the drillers, mud engineers, workover supervisors, well casing and cementing engineers and other key people and the procedural decisions and directions that these people take have an even greater impact on the probability that anomalous fluid flow or pressure changes (“kicks”) might develop inside the well bore.

Placing sufficient quantities of accurately formulated drilling mud into the well bore is the first line of defense against an oil spill or a blowout. It's definitely a plus for the whole operation if all or most crew members have some basic familiarity with the preparation of the drilling mud. Needless to say, an accurate and meticulously calculated mud-composition formula in the hands of a careless or  poorly trained mud-mixing personnel could lead to potentially dangerous situations. The mud engineer has to closely supervise the mud-preparation operation in such situations.

The well bore casing and casing cement serve primarily to maintain the integrity of the rock formations and to prevent the “non-producing” formations from being contaminated with the drilling fluid and the brines and/or petroleum that come from the “producing” formations. Casing failures and casing-cement failures could lead to a host of problems, not the least of which are oil spills and blowouts.  

Blow-out preventers (BOPs) are fairly sophisticated mechanical devices that are meant to plug or seal either the well bore (annulus) or the drill pipe channel. The two basic types are the annular and the ram BOPs. The ram type has four subtypes; namely, the pipe ram, blind ram, shear ram, and the blind shear ram.

The rig hands could significantly improve drill rig safety if they expend sufficient time and effort in adding to their stock knowledge of well-control procedures. Such familiarity can help the junior crew to correctly and efficiently carry out instructions during high-stress and time-constrained situations. Such familiarity also increases the observational and monitoring skills of junior crews, enabling them to quickly spot and identify a greater number of potentially dangerous situations and conditions and thus help provide relevant and timely information to the decision makers.

Sources:
1.https://en.wikipedia.org/wiki/Offshore_oil_spill_prevention_and_response

2. https://en.wikipedia.org/wiki/Well_control

3.http://www.bp.com/.../Deepwater_Horizon_Accident_Investigation_Report.pdf